Parallel plate electron multiplier having an inclined electric field and operative without a magnetic field



1966 T. A. CHUBB 3,278,751

PARALLEL PLATE ELECTRON MULTIPLIER HAVING AN INCLINED ELECTRIC FIELD AND OPERATIVE WITHOUT A MAGNETIC FIELD Filed Jan. 28, 1963 T zooov 1/ I C INVENTOR.

TALBOT A. CHUBB ATTORNEY United States Patent 3,278,751 PARALLEL PLATE ELECTRON MULTIPLIER HAV- ING AN INCLINED ELECTRIC FIELD AND 0P- ERATIVE WITHOUT A MAGNETIC FIELD Talbot A. Chubb, Arlington, Va., assignor t0 the United States of America as represented by the Secretary of the Navy Filed Jan. 28, 1963, Ser. No. 254,518 7 Claims. (Cl. 250-207) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to photomultipliers and more particularly to a parallel plate photomultiplier or parallel plate electron multiplier.

Heretofore, one type of electron multiplier has been made with a plurality of space dynodes each coated with a layer of secondary emissive material. These electron multipliers require particular care in arranging the dynode structure and a power supply of different potential for each of the separate dynodes. Another type is known as a magnetic electron multiplier which is made of parallel plates in combination with a magnetic field. The parallel plates are made of insulating material coated with a high resistance film of secondary emissive material, and an electric field is maintained both along the plates and between the plates. Thus both an electric and magnetic field is required'in the magnetic electron multiplier to accelerate secondary electrons along the parallel plates to a collector. Under the influence of both the magnetic field and electric field the primary and secondary electrons are guided along cycloidal paths between parallel plates which paths repeatedly intersect the more negative of the parallel plates thereby providing electron plate collisions and secondary electron multiplication.

It is an object of the present invention to provide a parallel plate electron multiplier operative without a magnetic field.

Another object is to provide a reliable, relatively inexpensive, and easily operable electron multiplier.

Still another object is to provide an efiicient detector for ultraviolet and high energy particles in which light or particles are converted into low energy electrons in an assembly in which electrons are multiplied into large numbers.

Yet another object is to provide a device which may be exposed to the presence of dry air without suffering damage;

Other and more specific objects of this invention will become apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawings in which:

FIG. 1 is a perspective view, partly in block form, schematically illustrating an electron multiplier constituting one embodiment of the present invention;

FIG. 2 illustrates modifications of the device shown in FIG. 1 and FIG. 3 is a plan view illustrating the electric field and secondary electron path between the parallel plates.

The present invention is directed to a photomultiplier comprising at least one fiat plate dynode having a conductive surface and means associated with the flat plate for producing an electric field gradient along the length of the plate. Extreme ultraviolet light or high energy particles striking a cathode surface or a continuation of the dynode surface produces electrons which are forced in steps along the length of the dynode to a collector electrode spaced from the end of the dynode surface. An

3,278,751 Patented Oct. 11, llfififi ice amplifier connected to the collector amplifies the signal from the collector electrode which is then recorded or directed to some other equipment depending on the intended use.

Now referring to the drawings there is shown by illustration in FIG. 1 one embodiment of a photomultiplier or electron multiplier made in accordance to the teaching of this invention. The multiplier includes a pair of parallel plates 11 and 12 separated from each other by about inch. The plates are made of glass or any other suitable insulating material which is coated on opposing faces with a thin conductive coating 13 such as tin oxide (SnO or any other secondary emissive material having a high resistance. Each plate is provided with a low resistance electrode 14, such as silver, at each end thereof which is electrically connected to a voltage source such that at the top of the plate the voltage is at a negative 2000 volts and at the bottom at a negative 65 volts. A battery source of 20 volts negative is connected across the plates such that the positive terminal of the battery is connected to electrode 14 on plate 11 and the negative side is connected to the electrodes 14 on plate 12. Thus the plate 11 will be positive with respect to plate 12.

.A plate anode'15 is disposed in substantially perpendicular relationship to the plates 11 and 12 with a small distance therebetween. An amplifier 16 is connected electrically with the anode and then the amplifier is connected with a recorder 17 or any other instrumentation to receive an amplified electron current signal collected by the anode.

In operation, the voltage supply of negative 20 volts is applied across each end of the plates 11 and 12 such that plate 11 is positive over the whole plate with respect to the plate 12. A voltage supply of negative 2000 volts is connected to the upper end electrode of plate 11 and a voltage supply of about negative 65 volts is connected to the bottom end of the plate. The application of a voltage to the end electrodes on plate 11 produces a small current flow throughout the length of the conductive resistance film on the face of plate 11 due to the high resistance of the conductive film. This current flow results in a uniform voltage drop across the conductive film on plate 11. An electric field is set up between the two plates in field lines 18, such as illustrated in FIG. 3 which are in a direction which causes electrons 19 to move toward plate 11.

Ultraviolet light or high velocity particles pass through the space between the plates striking the tin oxide dynode surface, or if desired, a cesium iodide photocathode. The incident light excites the atoms in the material, emitting electrons which are drawn toward the opposite plate. If the excited surface is on the plate 12, then the electrons will be drawn to plate 11 striking the dynode surface exciting secondary electrons. The secondary electrons are excited within the coating such that free electrons of difierent velocities in all directions are created within the coating or secondary emissive film. Only those electrons having sufiicient velocity in the proper direction will reach the surface of the coating and continue in a direction toward plate 12 between plates 11 and 12. The small electric field between the plates returns the electrons to plate 11 downstream of their point of emergence from the secondary emissive film toward the collector plate. The secondary electrons return to plate 11 with sufiicient energy to produce new secondary electrons which follows the process as pointed out above. The electrons are thus subjected to the combined action of the electric field along the plates 11 and 12 and the electric field produced by the potential applied between plates 11 and 12, with the result that repeated electron impacts occur on plate 11, most of which impacts involve electrons that have gained considerable energy from the electric field along the plates. These electrons produce new secondaries and thereby achieve the desired electron multiplication.

The force of the electric fields on the electrons emitted or released from plate 11 causes the electrons to travel in cyclic parabolic paths as shown in FIG. 3 impinging on different parts of the plate 11. The number of secondary electrons emitted increase exponentially in number of electrons for each parabolic cycle of travel to another part of the plate. In this manner, successively emitted secondary electrons travel across the length of the surface of the resistive coating on plate 11 in successive cyclic parabolic paths to multiply the number of secondary electrons initially emitted by the action from the incident light or high energy particle.

The modification shown by illustration in FIG. 2 comprises only one plate 21 which is provided with a photocathode coating of cesium iodide 22 on one face near one end of the plate and the remainder of the plate facing is coated with a high resistance conductive coating 23. Electrodes 24 are secured to each end of the plate and electrically connected to the adjacent coating on the coated face of the plate. A plurality of equally spaced electrically conductive wires 25 are secured to the plate and electrically connected with the conductive coating. The ends of the wires extend outwardly for about inch from the surface on a slant angle thereto toward the collector plate and then extend across the width of the plate in curved lines in a plane parallel to the plate. A negative supply source is connected to the end electrodes such that about negative 2000 volts is at the end adjacent the photocathode and about negative 65 volts is at the end electrode on the opposite end of the plate. An anode or collector electrode 26 is secured near the end having a potential of negative 65 volts. An amplifier is connected to the anode which receives an electrical signal from the collector due to incident electrons and which directs the amplified signal to a suitable recorder or any other desired instrumentation.

In operation of the device shown by FIG. 2, the applied voltage source produces a uniform field across the plate which has a voltage drop of uniform gradient across the length of the plate between the end electrodes. The electrical conductors 25 connected with the electrical coating arrive at the same potential as the plate at the point at which the conductor is connected to the plate, that is, the conductors will be at a less negative potential respectively, in order, from the end which is at 2000 negative potential to the end having negative 65 volts. A light source or high energy particle striking the cesium iodide cathode coating causes emission of electrons in accordance to the incident source. The electrons will be directed in a direction away from the plate in paths depending on the velocity and energy of the secondary electrons. The electrical conductors 25 are bent so as to provide an electric field pushing electrons back toward the plate, almost duplicating the electric field geometry produced by the effort of plate 12 in the previously described version of the invention. Since the plate and electrical conductor near the cathode are at a more positive potential than the cathode the secondary electrons will be pulled back onto the plate coating. The electrons striking the coated surface produces secondary electrons which are exponentially increased in numbers of electrons emitted by the surface. These secondary electrons will be directed away from the surface and again pulled back onto the surface by the less negative potential on the conductor across the plate surface. The electrodes 25 are more negative than the corresponding points on the plate surface, thereby providing an electric field component directed toward the plate. These electrons strike the surface releasing more secondary electrons which follow the pattern as described .above until the last electrons emitted by the plate are collected by the anode or collector. Current from the collector will be in proportion to the electrons collected and the electrons collected are dependent on the amount of radiation incident on the photocathode; therefore, from a measure of the current from the anode the radiation incident on the photocathode can be determined. An amplifier is electrically connected with the anode to amplify the signal produced by the collector which then feeds the amplified signal to a recorder or some other desired instrument.

Each of the devices can be used as a detector for a spectrograph and for many other uses. The device shown in FIG. 1 has a slit which will serve as an aperture for incoming radiation, therefore the parallel plate type needs no additional structure for directing radiation to be detected. The modification shown in FIG. 2 requires an aperture for directing radiation onto the cathode area of the device, therefore requiring additional outside structure for directing the radiation onto the photocathode.

Obviously many modifications and variations of the present invention are possible in the light of the aboveteachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An electron multiplier comprising (a) first and second closely assembled parallel flat plates of insulating material,

(b) a thin coating of high resistance material on opposing surfaces of said plates,

(c) an electrode secured on opposite ends of each of said plates and extending across the width thereof in electrical contact with said coating at each end of said plates,

(d) a first negative voltage source connected across said first plate to said electrodes on said first plate for producing a constant voltage across the width thereof with a voltage drop of uniform gradient along the length of said coating on said first plate,

(e) a second voltage source connected to the electrodes of each plate at the same ends of said first and second plates to produce a uniform electric field between the thin coating of high resistance material on the surface of each of said plates with said first plate positive with respect to said second plate,

(f) said electric fields forcing electrons emitted by the surface coating on said first plate back to the surface in a cyclic parabolic path along the length of said plate,

(g) a collector electrode arranged along one end of said plates in close relationship with the electrodes on the end of the plates to collect secondary electrons emitted by said plate and to produce an electrical current flow in accordance to the secondary electrons received by said collector.

2. An electron multiplier as claimed in claim 1 wherein the voltage supply connected to the electrodes on said first plate is about negative 2000 volts at the electrode on one end and negative 65 volts at the electrode adjacent to said collector plate.

3. An electron multiplier as claimed in claim 2 wherein said voltage supply connected between the electrodes on said first and second plates is about negative 20 volts.

4. An electron multiplier as claimed in claim 3 wherein the coating on said plates is tin oxide.

5. An electron multiplier comprising (a) a fiat plate of insulating material,

(b) a thin coating of high resistance material on one side surface of said plate,

(c) an electrode secured onto opposite ends of said plate and extending across the width thereof in electrical contact with said high resistance coating on the side surface,

((1) a negative voltage source electrically connected to said electrodes on the ends of said plate for producing a voltage drop of uniform gradient across the length of said coating on said plate,

(e) a plurality of equally spaced parallel electrodes connected at each end to said plate, extending outwardly therefrom for a distance and then across said plate in a plane parallel with said plate,

(f) said parallel electrodes adapted to arrive at the same potential as the plate at their point of connection thereof,

(g) said electrical potential on said parallel electrodes forcing electron emitted by said surface coating on said plate back to the surface in a cyclic parabolic path along the length of said plate, and

(h) a collector electrode positioned relative to one end of said plate to collect secondary electrons emitted by said plate and to produce an electrical current flow in accordance to the secondary electrons received by said collector.

6. An electron multiplier as claimed in claim 4 wherein References Cited by the Examiner UNITED STATES PATENTS 2,163,700 6/1939 Ploke et a1. 313-95 2,841,729 7/1958 Wiley 328-243 X 3,128,408 4/ 1964 Goodrich et al 250-207 FOREIGN PATENTS 884,059 7/ 1953 Germany.

RALPH G. NILSON, Primary Examiner. S. ELBAUM, Assistant Examiner. 

5. AN ELECTRON MULTIPLIER COMPRISING (A) A FLAT PLATE OF INSULATING MATERIAL, (B) A THIN COATING OF HIGH RESISTANCE MATERIAL ON ONE SIDE SURFACE OF SAID PLATE, (C) AN ELECTRODE SECURED ONTO OPPOSITE ENDS OF SAID PLATE AND EXTENDING ACROSS THE WIDTH THEREOF IN ELECTRICAL CONTACT WITH SAID HIGH RESISTANCE COATING ON THE SIDE SURFACE, (D) A NEGATIVE VOLTAGE SOURCE ELECTRICALLY CONNECTED TO SAID ELECTRODES ON THE ENDS OF SAID PLATE FOR PRODUCING A VOLTAGE DROP OF UNIFORM GRADIENT ACROSS THE LENGTH OF SAID COATING ON SAID PLATE, (E) A PLURALITY OF EQUALLY SPACED PARALLEL ELECTRODES CONNECTED AT EACH END TO SAID PLATE, EXTENDING OUTWARDLY THEREFROM FOR A DISTANCE AND THEN ACROSS SAID PLATE IN A PLANE PARALLEL WITH SAID PLATE, (F) SAID PARALLEL ELECTRODES ADAPTED TO ARRIVE AT THE SAME POTENTIAL AS THE PLATE AT THEIR POINT OF CONNECTION THEREOF, (G) SAID ELECTRICAL POTENTIAL ON SAID PARALLEL ELECTRODES FORCING ELECTRON EMITTED BY SAID SURFACE COATING ON SAID PLATE BACK TO THE SURFACE IN A CYCLIC PARABOLIC PATH ALONG THE LENGTH OF SAID PLATE, AND (H) A COLLECTOR ELECTRODE POSITIONED RELATIVE TO ONE END OF SAID PLATE TO COLLECT SECONDARY ELECTRONS EMITTED BY SAID PLATE AND TO PRODUCE AN ELECTRICAL CURRENT FLOW IN ACCORDANCE TO THE SECONDARY ELECTRONS RECEIVED BY SAID COLLECTOR. 